She said FitBit was “superior” to other ovulation monitoring methods, which vary in complexity, and may require women to record data, which is easy to forget.

The findings are only the pilot stage of a large-scale study that will examine resting heart rates in women of different ages and with varying lifestyles.

Women’s bodies change as a result of the menstrual cycle, leading to fluctuations in water retention, weight, metabolism, flexibility, temperature and eyesight.

Fertility expert Professor Allan Pacey, from Sheffield University, said there was nothing wrong with monitoring ovulation – but warned women not get too focused on only having sex on “fertile” days.

He said men can suffer “performance anxiety” if their partner puts too much pressure on them to have sex on these days.

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He said: “One of the biggest issues, I think, is men suffering from performance anxiety, or just being turned off, when women are fixated on a time or a date, a day or night which she has decided is the best time to try for a baby.

“There’s no problem with women using data if it helps them better plan things, but you don’t want to undo that benefit by sharing too much of that with him, and risk putting him off.

“You have a disconnection between couples, where the man is told you can’t ejaculate for a fortnight because two weeks on Tuesday, she reckons she is going to ovulate.